Pipe sizes do not refer to any physical dimensions. The outside diameter of each pipe or fitting must be measured and compared to the table for size identification. For example, a 1/4" NPT pipe thread has an outside diameter of .540 inches. Each thread size has a defined number of threads per inch (TPI). The 1/4" NPT pipe thread has 18 threads per inch. Both the TPI and OD of the thread are required for positive identification of thread size because several sizes have the same TPI.

Male threads:  Measure the outside diameter of the large portion of the thread at "A"; Find figure nearest this dimension in column 1 or 2 of chart. The dimension in column 3 will be your nominal pipe thread size.
Female Threads: Measure top diameter of thread at "B"; Find figure nearest this dimension in  column 1 or 2 of chart. The dimension in column 3 will be your nominal pipe thread size.

Two common pipe thread sizes exist, the tapered National Pipe Thread (NPT) and the straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM).  The NPT and NPTF threads are interchangeable with sealants such as Teflon tape. None of the other thread standards are interchangeable. Female NPT threads can be designated as "FPT" and male NPT threads can be designated as "MPT."